1. Field of the Invention
This invention is concerned with a four-way rotary proportional servo valve wherein the valving mechanism is characterized by low inertia and balanced axial forces.
2. Discussion of Related Art
Servo valves are used to control the operation of hydraulic devices such as fluid-driven bi-directional linear actuators. The servo system may be as simple as a manually-operated spool valve for delivering hydraulic fluid to one or the other of two input ports of the actuator at an operating frequency of one cycle per minute or less. Here, the operator himself (herself) comprises the feedback loop and the system delay time is a function of the actuator loading and the operator's personal reaction time. On the other hand, the valve-control system may encompass a sophisticated programmed closed-loop feedback control employing high gain and negative feedback operating at hundreds of cycles per second. For low-frequency applications such as an hydraulically-powered earth mover or a backhoe, the servo-system response time is of little concern. In more complex systems that employ high flow rates, multistage valves are used. Multistaging introduces unwanted system delays that may lead to instabilities at high frequencies.
In a seismic exploration survey, a hydraulic vibrator, coupled to a ground-contacting base plate, is used to shake the ground at designated stations thereby to inject a powerful chirp signal into the earth at each of those stations. The frequency of the chirp signal ranges from about 5 Hz to about 160 Hz or more over a time span on the order of eight to sixteen seconds. More than one vibrator is normally used at each station during the progress the seismic survey. It is essential that the vibrator output signals of the respective vibrators must be accurately synchronized within less than a millisecond in high resolution geophysical surveys.
A typical well-known spool-type servo valve used by the seismic exploration community is described in U.S. Pat. No. 4,593,719 issued Jun. 10, 1986 to W. B. Leonard. Although this valve is an industry standard, problems arise at high frequencies by reason of the mass of the valve spool.
In an effort to evade the problems of inertia that plague spool-type servo valves, rotary servo valves have been developed. In that type of valve, a rotating orifice plate periodically interrupts the fluid flow from a source, through a distributor which directs the flow alternately between the two pressure inputs of an actuator. The angular velocity of the orifice plate controls the frequency. Because the orifice plate always rotates in one direction, the inertial effect presumably does not arise provided the angular velocity remains constant.
One type of rotary valve is disclosed in U.S. Pat. No. 4,442,755, issued Apr. 17, 1984, to Marek L. Rozycki, for a Power Stage Servo Valve for a Seismic Vibrator. The valve includes a relatively large-diameter rotatable orifice plate and a stationary orifice plate. The configuration of the orifices is such that the valve opens more quickly than it shuts off when the orifice plate rotates in one direction at a desired angular frequency. If the orifice plate were to rotate at a constant angular velocity, and hence interrupt the fluid flow at a constant frequency throughout a duty cycle, the inertia problem would not exist. among the problems with the Rozycki valve are first, that it cannot implement a pseudo-random sweep. Second, the orifice shapes a optimized for a particular loading and thus is not very flexible in its application.
As is well known in the art, inertial forces increase as the fourth power of the diameter of a rotary throttling device. A smaller diameter for the control member would dramatically reduce the inertial forces. U.S. Pat. No. 4,977,816 issued on Dec. 18, 1990 to W. Kuttruf for a Hydraulic Motor Control System with a Rotating Servo Valve provides a rotary valve which employs a rotating piston. The piston is in the form of a rod along which are cut axially disposed recesses on the outer surface of the rod. Although the diameter of the control member has been reduced substantially as compared to the Rozycki device, the recesses provide fluid passageways whose cross-sectional area is severely restricted such that the volume of fluid flow per unit time is inadequate for a seismic vibrator where flow rates in excess of 100 gallons per minute at systems pressures of 3000 psi are commonplace. The fluid flow restriction could be reduced by making the recesses deeper but that would be at the expense of reducing the torsional stiffness of the rotating piston. In high-frequency-response duty cycles, torsional resonances that lie within the useful seismic frequency band create stability problems, introduce spurious seismic responses due to unwanted signal resonances and lead to mechanical fatigue with ultimate failure of the valve.
U.S. Pat. No. 5,014,748, issued May 14, 1991 to T. Nogami, teaches a rotary valve in which a motor drives a disk inside of a casing. The relative position of the disk slots and the casing slots determine the opening area of the control orifices. This technology, while perhaps able to provide reasonable response time in low flow valves, the mechanism does not scale. In high flow applications where rapid response is critical, the inertia load for a disk becomes the limiting factor. The moment of inertia for a disk increases as the radius raised to the fourth power as earlier mentioned. For a disk sturdy enough not to deform under high pressure situations, the disk cannot be made arbitrarily thin. Therefore, higher power may be required to accelerate the valve having the geometry disclosed in the '748 patent than the rotary valve of this invention next to be described.
There is a need for an economical, high-frequency-responsive, low-inertia servo valve that will be mechanically stable, that will provide a high fluid-flow rate with a pressure drop less than about 1000 psi and that will be characterized by a robust mechanical design.